Ultra-high molecular weight, linear polyethylene, having molecular weight of at least about 1.times.10.sup.6 and density of between 0.92 to 0.99 has been known for some time. On account of its outstanding physical properties including toughness, impact strength, abrasion resistance, low coefficient of friction, combined with excellent resistance to attack by solvent and corrosive chemicals, it has been found useful in demanding applications including vibration dampener pads, hydraulic cyclinders, mallet heads, flexible drive couplings, gears, belt and chain guides, door stops, bumpers, machinery carriages, conveyor equipment components, bearings, and especially in textile machine parts such as loom pickers, drop box pickers, lock straps, sweep sticks, lock connectors, pick arm stop bolts, and the like. Its use, however, has been limited because of fabrication difficulties. Due to its high molecular weight it has very high melt viscosity, which makes it difficult or impractical to fabricate it by conventional thermoplastic processing techniques, such as injection molding. Not only that, it is also sensitive to melt shear, which causes melt fracture and physical degradation.
Previously available methods for fabricating ultra-high molecular weight polyethylene include compression molding, flow molding, transfer molding and ram extrusion, all of which are cumbersome and slow as compared to melt extrusion usually employed for thermoplastic materials. Compression molding of such ultrahigh molecular weight polyethylene requires maintenance of the molded part under elevated temperature and pressure in the mold for extended periods of time to melt the polyethylene and to allow it to melt flow into a solid article free of voids, and then cooling it under pressure while confined in the mold. Equipment required for such operation is complex and expensive and production rates are exceedingly slow. A plurality of identical molds are usually required in order to permit acceptable production rates. Therefore, there is a need for a simplified molding process for forming articles of ultra-high molecular weight, linear polyethylene.
It is an object of the present invention to provide a new method for forming solid articles of ultra-high molecular weight, linear polyethylene.